You can use Japanese quite easily. You have to install the "Japanese Language Kits", an addition at the original Mac OS 9 CDs. The 2nd thing you have to do, is to use Japanese fonts.
Here is an "howto" in Japanese itselve: http://www.kenkyuu.net/computer-01.html
It seems very straightforward, but when I boot from the MacOs9Lives CD, all I find is an application called "Mac OS Restore", not "Mac OS Install". And Mac OS Restore seems to want to do the whole business of erasing my disk and reinstalling OS 9, which makes me feel quite cautious. I wonder if this MacOS9Lives image even contains the files mentioned in that guide?
Guide fed to a translator for English references, and because I find skimming through long Japanese documents quite difficult too
In this totally useless and frustrating post, I'll note that I sold a G3 Lombard with a Japanese-character keyboard just a few months ago…
I should have listed it here first but it went almost overnight and I forgot… Sorry.
Oh that's alright, I couldn't justify buying any more computers, and we're probably not on the same continent either.
It's also very different to type with a Japanese layout keyboard because on them, you're typing one character per key, while people without a Japanese keyboard are used to typing syllables as they are, and only once the syllable has been typed, the Japanese syllable character for it appears, and by pressing the space bar, you cycle through transformations and alternatives for it. I'd have to completely re-learn typing if switching to a Japanese keyboard!
Also, changing physical keyboards is really difficult because the punctuation characters are usually mapped to completely different keys; This has partially affected the way people from different countries have typed ASCII smilies over the years.